The present invention is related to a novel protein p140 polypeptide which is a key protein involved in the signal transmission system of insulin; method for preparation of it; DNA encoding the said polypeptide; vector derived the said DNA; host cells transformed the said vector; antibody of the said polypeptide; pharmaceutical composition containing the said peptide or antibody; method for the prevention and/or treatment of diabetes, which is characterized by tyrosine phosphorylation of the said protein p140 (to be quoted henceforth as phosphorylation in the present detailed specification); agent for the prevention and/or treatment for the currently said the prevention and/or treatment method; agent for the prevention and/or treatment of diabetes, which is characterized by containing a compound which can tyrosine phosphorylate of protein p140, as active ingredient and the screening methods of the said prevention and/or treatment agent.
Diabetes, an abnormal metabolic disease, is induced by a defect in the mechanism of glucose metabolism.
Under normal conditions, glucose metabolism occurs as follows: Carbohydrates, consumed in the form of food, are digested to glucose in the intestines prior to absorption into the circulatory system. Pancreatic xcex2 cells respond to an increase in the blood glucose level by secreting insulin, which in turn stimulates the target peripheral tissues (muscles and liver) to decrease the blood glucose level by enhancing tissue absorption of the blood glucose followed by conversion to glycogen for storage.
Depending on the causative factors, diabetes is classified into two major categories; insulin dependent diabetes mellitus (IDDM) and non-insulin dependent diabetes mellitus (NIDDM). IDDM (Type I diabetes) is a pathological condition where insulin is not secreted or insufficient even on secretion by pancreatic xcex2 cells responding to an increase in the blood glucose level induced by food consumption. It has been known that destruction of xcex2 cells of the pancreatic islets induces IDDM. The current therapy employs supplementation of insulin from exogenous sources.
NIDDM (Type II diabetes) is a pathological condition where the feedback mechanism of peripheral tissues is dysfunctional and is ineffective in decreasing the blood glucose level although normal insulin secretion occurs within the living system. In the United States of America, NIDDM is said to be a common disease; 5% of the population exceeding 40 years of age suffer from NIDDM. Causative factors involved in this disease have yet to be elucidated.
Elucidation of the etiology of NIDDM; namely, clarification of the insulin-induced glucose uptake mechanism in peripheral tissue cells is, however, unclear as current knowledge on information transmission mechanism of insulin remains limited and unestablished.
Insulin secreted from the pancreatic islets binds with insulin receptors on the cell membrane of peripheral tissue cells. With regards to post-binding information transmission, the phosphorylase cascade and second messenger theories are the current topics of research.
Briefly, these two theories can be accounted as follows:
Phosphorylase Cascade Theory:
When insulin binds with the insulin receptor xcex2 subunit, the xcex1 subunit existing on the inner cell membrane triggers phosphorylation accompanied by activation of the tyrosine kinase site within the receptor. Phosphorylation of substrates by the latter enzyme produces three different proteins. One is composed of 1,235 amino acids and has a molecular weight of 185 kD corresponding to the insulin receptor substrate-1 (IRS-1). On tyrosine phosphorylation of IRS-1, the phosphorylase for phosphatidylinositol, PI1-kinase, binds against and activates the complex. Post-binding events related to information transmission that concerns localization of glucose transporter within the membrane and membrane ruffling have yet to be established. Other than IRS-1, the existence of two protein substrates (Shc and PTP-1C) has been confirmed. However, the follow-up mechanism(s) has not been completely accounted for.
Second Messenger Theory:
When insulin binds against the insulin receptor, phospholipase C is specifically activated to degrade phosphatidylinositol glycan (PIG) to produce inositolglycan (IG) and diacylglycerol (DAG) by hydrolysis. Although IG has been reported to display various insulin-like effects, the typical glucose uptake effect has yet to be demonstrated.
However, when protein kinase C is activated by DAG, localization of protein kinase C within the cell membrane has been known to be promoted. This implicates that DAG sequentially phosphorylates inner membrane proteins to finally trigger the glucose uptake. However, this implication remains hitherto unclear.
Although the two different schools of thought have hitherto prevailed, initial stages of the post-binding events related to information transmission can only be explained in part by either theory.
According to Copper et al. in 1988 the hormone, amylin, is released from b pancreatic cells that similar to those that secret insulin when hyperglycemia prevails. Based on their findings that amylin inhibited the action of insulin, they revealed that the hormone might be used as an insulin antagonist. A follow-up report in 1991 indicates that the excessive use of amylin in transgenic mice induces NIDDM. However, the relationship of amylin with insulin information transmission remains hitherto unexplored.
The inventors of the present invention focus on the insulin antagonistic properties of amylin. With persistent research activities conducted on the effects of amylin on the insulin information transmission system, the inventors first identified the inhibition site of amylin in regulating the insulin information transmission system and discovered the key proteins, phosphorylated protein 140 and 70 (pp140 and pp70), related to this phenomenon. The present invention reveals clearly the structures of said proteins (DNA base sequences and amino sequences) and elucidation of their functions to totally complement the hitherto deficiently explained insulin information transmission phenomenon.